In wastewater treatment plants that have a biological stage, it is important that the substances dissolved in the wastewater be coagulated quantitatively and that they be flocculated in a stable manner along with the particulate substances and the microorganisms that are present. This is necessary to ensure that the sludge flakes sediment out in the subsequent final clarification process and are thereby separated from the purified wastewater, which can be discharged into the environment.
Therefore the goal is that solids, as well as the dissolved carbon, nitrogen, and phosphorus contents, be eliminated from the wastewater. The maximum permissible concentrations of these substances in the discharge of the wastewater treatment plant are defined by government standards. In addition, for the most important substance groups, purification performance expressed in percent—in other words, elimination rates relative to the amount of material fed into the wastewater treatment plant—are specified.
Critical factors in this process are quantitative coagulation, stable flocculation, as well as the rapid sedimentation of the sludge flakes, so that the process continues to occur uninterrupted in a stable manner and so that, as a result, high purification performance is achieved.
Traditionally, iron compounds (iron sulfate and iron chloride) as well as aluminum sulfate, are used as precipitation and flocculation agents in wastewater treatment plants, and in particular also for the elimination of phosphorus. The dose levels are typically in the range of from 50 to 250 ppm (ml per m3 wastewater). These agents make the sludge heavier in order to improve sedimentation, and they also precipitate phosphate chemically. It has been shown that stable long-term operation is theoretically possible. The disadvantages include high consumption of chemicals accompanied by a corresponding load on the sludge that is to be sedimented out and disposed of and the adverse effect on the biological activity of the activated sludge as well as a high sensitivity to load variations and temperature variations in the wastewater treatment plant.
For at least 10 years, precipitating and flocculating agents based on polyaluminum have been present in the market. These agents are oligomeric molecule clusters of AlCl3 and H2O with a cluster size in the range of approximately 13. They have the advantage of greater effectiveness with lower doses, and they also remain effective at low water temperatures. However, the previously available products based on polyaluminum have problems because of their low storage stability at the user's facility and because of the low stability and reliability of the biological purification process. It is frequently observed that after a plant begins to use a certain polyaluminum precipitating agent for a certain time—for example for some months—the purification operation will suddenly experience poor performance, even amounting all the way to a complete stoppage. Changing to a different product can lead to a temporary recovery, but this recovery does not last.
Document DE 23 34 766 B2 describes the treatment of wastewater containing emulsified substance using a composition of matter that contains aluminum chloride and a macromolecular flocculent, as well as additional silicic acid and silica gel as the absorbent. The flocculent may be a polyamine based compound, which is separated from the wastewater and which has the other components added to it.
In DE 295 093 A5, in order to split oil-in-water emulsions, a pre-cleaving operation is performed with, for example, polyaluminum salts, and a primary cleaving operation is performed with a copolymer of polyacrylonitriles and hydroxylamine. Aluminum chloride is also considered as a cleavage agent, but it is less recommended.
Patent Abstracts of Japan No. 10017857 describes a mixture of chemicals that is “packed” into a water-soluble container, but whose purposes are not precisely stated and that contains, among other things, polyaluminum chloride and a polymeric flocculent.
WO 99/35090 A2 describes a process for preparing a polyaluminum chloride or chloride sulfate flocculent in order to react an aluminum compound with acid aqueous medium containing chloride ions and/or sulfate ions, magnetic substance—and carbonate-containing minerals, and calcium carbonate-containing minerals. The reaction produces a polyaluminum-calcium-magnesium-chloride or chloride sulfate. The composition of matter may also contain a cationic polymer.